In general, carbon fibers are light weight fiber materials having high tensile strength and elasticity, and can be classified into the types of high tensile strength carbon fiber, wherein the tensile modulus is from about 20,000 to 28,000 Kg/mm.sup.2, and high elasticity carbon fiber, wherein the tensile modulus is at least about 30,000 Kg/mm.sup.2. Thus, depending upon the characteristics thereof, it can be used as a reinforcing material for various plastic materials for use in production of spacecraft structural materials, car and industrial machine parts, and so forth. These carbon fibers, however, are required to have good adhesive properties to matrix resins as well as high strength and heat-oxidation resistance.
In order to increase the adhesion of carbon fibers to resins, it is usually necessary to surface-treat the carbon fiber, and various methods have heretofore been proposed. Of these methods, a so-called electrolytic processing method wherein a current is passed through a carbon fiber in an aqueous solution of an electrolyte, such as sodium hydroxide, sulfuric acid, or phosphoric acid, has been considered to be advantageous from an economic standpoint. Such electrolytic processing methods are described, for example, in Japanese Patent Publication No. 40119/72 and U.S. Pat. Nos. 3,671,411 and 3,759,805.
These electrolytic processing methods, however, are liable to deteriorate the inherent strength and heat-oxidation resistance of the carbon fiber, although they do improve the adhesion of the carbon fiber to resins. In particular, it has been found that application of such known electrolytic processing methods to the so-called high strength type of carbon fiber having a tensile modulus of from about 20,000 to 28,000 Kg/mm.sup.2 results in a great deterioration of its inherent high strength and heat-oxidation resistance. As has already been described above, however, a carbon fiber is generally required to exhibit not only good adhesion to resins, but also at the same time to have high tensile strength and heat-oxidation resistance, in view of the applications in which it is used.